This invention relates to testing and measurement of optical and electrical signals.
Single-shot transient signals having a frequency of 6 GHz or less can be measured using a real-time oscilloscope, such as the Tectronics Inc. TDS6604 oscilloscope, manufactured by U.S. Tectronix Incorporated, Beaverton, Oreg. Another instrument for measuring a single-shot transient signal is a streak camera, which can cover frequencies ranging from the MHz-range up to about 100 GHz, such as the Hamamatsu C5680 streak camera, manufactured by Hamamatsu, Japan. Currently, there are no other instruments available for measuring a transient signal with a frequency response up to 100 GHz.
The upper frequency limit for the single-shot transient signals that can be measured with a real-time oscilloscope is determined by the sampling rate of the real-time oscilloscope. Higher frequency responses, up to 50 GHz or higher, can easily be measured with a sampling oscilloscope. However, a sampling oscilloscope can only measure repetitive signals and not single-shot transient signals.
The above-mentioned streak camera works as follows. An electrical or optical signal is converted into a low-energy, secondary electron beam by letting the input electrical or optical beam strike a photo-multiplier tube (PMT). The low-energy secondary electron beam has the same energy distribution as the original signal. The low-energy secondary electron beam passes between a pair of deflecting electrodes that deflect the secondary electron beam as the secondary electron beam passes between the electrodes. The secondary, deflected electron beam then strikes a photon receiver, such as a charge-coupled device (CCD) camera. The CCD camera converts the energy distribution in the secondary electron beam, and consequently in the original electrical or optical signal, to a distribution that can be displayed. Presently, the streak camera is the only available type of measurement instrument permitting single-shot transient signal measurement for signals in the pico-second range. A streak camera is a very expensive measurement instrument and can only perform single-channel measurements.
In the telecommunications area, a common task is to ascertain how the temporal shape of a pico-second optical signal changes when the optical signal passes through an optical fiber over a long distance. The bitrate in optical transmission lines in telecommunication systems is typically 10 Gb/s, or in some cases even 40 Gb/s. Since signal deterioration may cause the error-bit-rate to increase, it often is necessary to understand how various dispersions along the optical path affect the pulses when the optical signals pass through a long route of several hundreds or thousands of kilometers fiber, as well as various optical devices and amplifiers. Therefore, a transient signal oscilloscope, which shows the real-time waveform of the signal, with frequency responses from 10 Gb/s to 40 Gb/s will be a very useful tool for this application.
So-called optical recirculating loops have been used for the simulation of long-haul optical transmission lines. One example of such use of a single-channel recirculating loop is given on page 455 of xe2x80x9cErbium-Doped Fiber Amplifiers: Principles and Applicationsxe2x80x9d by Emmanuel Desurvire, John Wiley and Sons, March, 1994.
In general, in one aspect, this invention provides methods, apparatus, and systems, including computer program products, implementing and using techniques for measuring multi-channel single-shot transient signals. A signal acquisition unit receives one or more single-shot transient pulses from one or more transient signal sources. An optical-fiber recirculating loop reproduces the one or more received single-shot optical pulses to form a first multi-channel pulse train for circulation in the recirculating loop, and a second multi-channel pulse train for sampling and display on a sampling and display device. The optical-fiber recirculating loop also optically amplifies the first circulating pulse train to compensate for signal losses and performs optical multi-channel noise filtration.
Advantageous implementations can include one or more of the following features. The signal losses can include losses caused by split-off from the first multi-channel pulse train in the recirculating loop. The transient pulses can be optical multi-channel single-shot pulses. The signal acquisition unit can select the pulses using a gating circuit. The signal acquisition unit further can time multiplex the selected pulses using optical delay lines of different lengths. The signal acquisition units can wavelength multiplex the selected pulses using an nxc3x971 optical add/drop multiplexer. The measurement instrument can be used for multi-channel signal reproduction. The optical fiber recirculating loop can perform noise filtration using two acoustic modulators, two nxc3x971 OADMs, and n delay lines of different length. The display device can be a correlator. The display device can be a sampling oscilloscope. The signal acquisition unit can select one or more targeted electrical single-shot pulses from the first and the second pulse trains using a gating circuit. The single-shot pulses can be electrical multi-channel single-shot pulses and the signal acquisition unit can includes one or more modulators and one or more bias setting circuits that can convert the electrical pulses into optical pulses. The optical-fiber recirculating loop can perform optical multi-channel noise filtration using two acoustic modulators, two nxc3x971 OADMs, n delay lines of different length; and realign the timing of the multi-channel pulse trains, so that the acoustic modulators, and the OADMs permit recovery of the original time sequence of the first and second pulse trains for display on a sampling oscilloscope or other correlator. The frequency of the one or more single-shot pulses can be less than 100 GHz.
The invention can be implemented to realize one or more of the following advantages. A measurement instrument is provided that is based on different physical principles than current measurement instruments, through the use of fiber optical technologies. The high-frequency response of optical fibers can be used to convert single-shot multi-channel signals into a repetitive multichannel pulse train, thereby allowing a sampling oscilloscope to be used for extracting the shape of the original signals. Such a multi-channel single-shot instrument, based on an optical-delay line recirculating loop, is complete new. The technical difficulties associated with high-speed sampling are greatly reduced. Electronic components used by the measurement system can operate at low speed, which reduces the overall cost of the instrument as compared to conventional measurement instruments using high-speed electronic components. Dispersions and other difficulties associated with fibers can be compensated for in much more efficient ways, such as through dispersion compensation, as compared to coaxial cables.
The temporal distribution of an optical or electrical multi-channel transient signal, in particular for the 10 GHz to 50 GHz frequency response range, can be measured. The frequency response in 10 Gb/s to 40 Gb/s transmission lines can be measured, which is important in the telecommunications area. A significant reduction of noise produced by an Erbium-doped fiber amplifier can be achieved in the multi-channel measurement instrument, which permits more than a 1000-fold reproduction of the original multi-channel signals. A user-friendly measurement system is provided in which the manual setting procedures have been reduced to a minimum compared to conventional measurement instruments.